A purr is a tonal fluttering sound made by some species of

felid Felidae () is the family of mammals in the order Carnivora colloquially referred to as cats, and constitutes a clade. A member of this family is also called a felid (). The term "cat" refers both to felids in general and specifically to the do ...

s and two species of genets. It varies in loudness and tone among species and in the same animal. Felids are a family of mammals that belong to the order

Carnivora Carnivora is a monophyletic order of placental mammals consisting of the most recent common ancestor of all cat-like and dog-like animals, and all descendants of that ancestor. Members of this group are formally referred to as carnivorans, ...

and are informally known as cats. This designation includes larger, outdoor cats and the domestic cat (''Felis catus''). Genets are members of the genus ''Genetta'' and are slim animals with features similar to cats. Their features include retractile claws, leopard-like spotted fur and the raccoon-like mask and ringed tail. Although true purring is exclusive to

s and

viverrids Viverridae is a family of small to medium-sized, feliform mammals. The viverrids () comprise 33 species placed in 14 genera. This family was named and first described by John Edward Gray in 1821. Viverrids occur all over Africa, southern Europe, ...

, other animals such as

raccoons The raccoon ( or , ''Procyon lotor''), sometimes called the common raccoon to distinguish it from other species, is a mammal native to North America. It is the largest of the procyonid family, having a body length of , and a body weight of . ...

produce vocalizations that sound similar to true purring. Animals that produce purr-like sounds include

mongooses A mongoose is a small terrestrial carnivorous mammal belonging to the family Herpestidae. This family is currently split into two subfamilies, the Herpestinae and the Mungotinae. The Herpestinae comprises 23 living species that are native to so ...

bear Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the Nort ...

kangaroo Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...

wallabies A wallaby () is a small or middle-sized macropod native to Australia and New Guinea, with introduced populations in New Zealand, Hawaii, the United Kingdom and other countries. They belong to the same taxonomic family as kangaroos and som ...

wallaroo Wallaroo is a common name for several species of moderately large macropods, intermediate in size between the kangaroos and the wallabies. The word "wallaroo" is from the Dharug ''walaru'', and not a portmanteau of the words "kangaroo" and "wa ...

badger Badgers are short-legged omnivores in the family Mustelidae (which also includes the otters, wolverines, martens, minks, polecats, weasels, and ferrets). Badgers are a polyphyletic rather than a natural taxonomic grouping, being united by ...

fox Foxes are small to medium-sized, omnivorous mammals belonging to several genera of the family Canidae. They have a flattened skull, upright, triangular ears, a pointed, slightly upturned snout, and a long bushy tail (or ''brush''). Twelve sp ...

es,

hyena Hyenas, or hyaenas (from Ancient Greek , ), are feliform carnivoran mammals of the family Hyaenidae . With only four extant species (each in its own genus), it is the fifth-smallest family in the Carnivora and one of the smallest in the cla ...

rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit specie ...

squirrels Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels (including chipmunks and prairie dogs, among others), and flying squirrels. Squ ...

guinea pig The guinea pig or domestic guinea pig (''Cavia porcellus''), also known as the cavy or domestic cavy (), is a species of rodent belonging to the genus ''Cavia'' in the family Caviidae. Breeders tend to use the word ''cavy'' to describe the a ...

tapirs Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inha ...

, ring-tailed

lemurs Lemurs ( ) (from Latin ''lemures'' – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagas ...

and

gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...

s while eating. Animals purr for a variety of reasons, including to express happiness or fear, and as a defense mechanism. It has also been shown that cats purr to manage pain and soothe themselves. Purring is a soft buzzing sound, similar to a rolled 'r' with a fundamental frequency of around 25 Hz. This sound occurs with noticeable vibrations on the surface of the body, varies in a rhythmic pattern during breathing and occurs continuously during inhalation and exhalation. The intensity and length of the purr can also vary depending on the level of arousal of the animal.

Mechanism

The mechanism by which cats purr is an object of speculation, with different theories proposed. An early theory was that purring is a hemodynamic process where sound is produced as the blood runs through the thorax. There is a unique " neural oscillator" in the cat's brain of uncertain significance. Although the mechanism has not yet been fully explained, recent studies have inferred it could be the result of oscillatory mechanisms in the central nervous system. Studies have also shown that purring can be caused through electrically stimulating the infundibular region of the cat's brain, suggesting central control.

Vocal folds/laryngeal muscles

One hypothesis, backed by

electromyographic Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyog ...

studies, is that cats produce the purring noise by using the

vocal folds In humans, vocal cords, also known as vocal folds or voice reeds, are folds of throat tissues that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speech ...

or the muscles of the

larynx The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

to alternately dilate and constrict the

glottis The glottis is the opening between the vocal folds (the rima glottidis). The glottis is crucial in producing vowels and voiced consonants. Etymology From Ancient Greek ''γλωττίς'' (glōttís), derived from ''γλῶττα'' (glôtta), v ...

rapidly, causing air vibrations during inhalation and exhalation. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong

harmonics A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...

Degree of hyoid ossification

No cat can both purr and roar. The subdivision of the Felidae into "purring cats" (

Felinae The Felinae are a subfamily of the family Felidae. This subfamily comprises the small cats having a bony hyoid, because of which they are able to purr but not roar. Other authors have proposed an alternative definition for this subfamily: as c ...

) on one hand and "roaring cats" ( Pantherinae) on the other goes back to Owen and was definitively introduced by Pocock, based on whether the

hyoid bone The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical ver ...

of the

is incompletely ("roarers") or completely ("purrers")

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in th ...

. However, Weissengruber et al. argued that the ability of a cat species to purr is not affected by the anatomy of its hyoid. The "roaring cats" (lion, ''Panthera leo''; tiger, ''P. tigris''; jaguar, ''P. onca''; leopard, ''P. pardus'') have an incompletely ossified hyoid, which, according to this theory, enables them to roar but not to purr. However, the

snow leopard The snow leopard (''Panthera uncia''), also known as the ounce, is a felid in the genus ''Panthera'' native to the mountain ranges of Central and South Asia. It is listed as Vulnerable on the IUCN Red List because the global population is es ...

(''Uncia uncia'', or ''P. uncia''), as the fifth felid species with an incompletely ossified hyoid, purrs. All remaining species of the family Felidae ("purring cats") have a completely ossified hyoid, which enables them to purr but not to roar. Based on a technical acoustic definition of roaring, the presence of this vocalization type depends on specific characteristics of the vocal folds and an elongated vocal tract, which is rendered possible by an incompletely ossified hyoid.

Frequency, amplitude, and respiratory variation

*Domestic cats purr at a

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz The hertz ...

of 20 to 30 vibrations per second. *Eklund, Peters & Duthie, comparing purring in a cheetah (''Acinonyx jubatus'') and a domestic cat (''Felis catus'') found that the cheetah purred with an average frequency of 20.87 Hz (

egressive In human speech, egressive sounds are sounds in which the air stream is created by pushing air out through the mouth or nose. The three types of egressive sounds are pulmonic egressive (from the lungs), glottalic egressive (from the glottis), ...

phases) and 18.32 Hz (

ingressive In phonetics, ingressive sounds are sounds by which the airstream flows inward through the mouth or nose. The three types of ingressive sounds are lingual ingressive or velaric ingressive (from the tongue and the velum), glottalic ingressive ( ...

phases), while the much smaller domestic cat purred with an average frequency of 21.98 Hz (egressive phases) and 23.24 Hz (ingressive phases). *Schötz & Eklund studied purring in four domestic cats and found that the fundamental frequency varied between 20.94 and 27.21 Hz for egressive phases and between 23.0 and 26.09 Hz for ingressive phases. Schötz & Eklund also observed considerable variation between the four cats as regards relative amplitude, duration and frequency between egressive and ingressive phases, but that this variation occurred within the same general range. *In a follow-up study of purring in four adult cheetahs, Eklund, Peters, Weise & Munro found that egressive phases were longer than ingressive phases in all four cheetahs. Likewise, ingressive phases had a lower frequency than egressive phases in all four cheetahs. Mean frequency were between 19.3 Hz and 20.5 Hz in ingressive phases, and between 21.9 Hz and 23.4 Hz in egressive phases. Moreover, the amplitude was louder in the egressive phases in four cheetahs. *Eklund & Peters compared purring in adult, subadult and juvenile cheetahs and reported that while there was considerable variation across most of the parameters analyzed (amplitude, phase duration, cycles per phase and fundamental frequency) – mainly attributable to degree of relaxation/agitation in the animals resting or playing– previously reported observations that ingressive phases tend to be lower in frequency were largely confirmed. There were no major differences in these parameters as a function of age.

Purpose

In domestic cats, many signals that occur when interacting with humans seem to originate from when the animal was dependent on the mother. Cats have been observed to purr for most of their lifespan, starting from when they were young and suckling from their mother. Purring may have developed as an evolutionary advantage as a signalling mechanism of reassurance between mother cats and

nursing Nursing is a profession within the health care sector focused on the care of individuals, families, and communities so they may attain, maintain, or recover optimal health and quality of life. Nurses may be differentiated from other health ca ...

kittens. Post-nursing cats often purr as a sign of contentment: when being petted, becoming relaxed, or eating. Some purring may be a signal to another animal that the purring cat does not pose a threat. In addition to this, cats have been shown to have different sounding purrs depending on the situation. Purring sometimes seems to be a way for cats to signal their caretakers for food. This purring has a high-frequency component not present in other purrs. These are called solicitation purrs (when the cat is looking for something) and non-solicitation purrs (when the cat is not looking for something), and the two are distinguishable to humans. In a study, 50 humans were subjected to playbacks of purrs recorded in solicitation and non-solicitation situations at the same amplitude. Humans regularly judged the solicitation purrs as less pleasant and more urgent than the non-solicitation purrs. This variety of purring seems to be found more frequently in cats in a one-to-one relationship with a caretaker. Similarities have previously been drawn between an infant crying and the isolation cry of domestic cats. The high frequency aspect of the purr can subtly exploit humans' sensitivity to these cries, which makes it more difficult to become habituated and ignore this. Using sensory biases in communication between species provides the signalers (cats) with a productive way to increase the standard of care received. Cats often purr when distressed or in pain, such as during labor. In cats, there are three stages of labor. In the first stage, the uterus begins to contract, the cervix relaxes and the water breaks. During this stage, the cat begins to purr which is hypothesized to be a self-relaxation technique. This purring may trigger a cat's brain to release a hormone which helps it in relaxing and acts as a

painkiller An analgesic drug, also called simply an analgesic (American English), analgaesic (British English), pain reliever, or painkiller, is any member of the group of drugs used to achieve relief from pain (that is, analgesia or pain management). It i ...

. There is also some evidence that the frequency at which cats purr can help promote bone density and hasten healing.

References

External links

Why do cats purr? Why do Cheshire cats grin?

Robert Eklund's Purring Page
{{Authority control Animal sounds Cat behavior Ethology